Sometimes Photoshop will start exhibiting weirdness. Palettes may disappear, tools may start performing contrary to our wishes (or not at all). Options may seemingly change of their own volition. These and many other symptoms are commonly caused by the preferences file becoming corrupt. Anytime you suspect that things in PS aren't as they should be, the first thing to try is deleting your preferences file. When you do this PS will automatically create a new file with the factory defaults. The easiest way to do this is to hold down the Alt, Ctrl, and Shift keys (Mac Command, Option, Shift) immediately after starting Photoshop. A dialog box will appear asking if you wish to delete the preferences file. Alternatively, you can simply search your hard drive for a *.psp file and delete it.